Hot-melt adhesives are known and have been used in industry for various applications such as product assembly. Examples of such applications are in the packaging industry in the fabrication of corrugated paperboard, cardboard case sealing and carton closing operations. The adhesive is generally applied by a hot-melt applicator which consists of a heated reservoir for maintaining the adhesive in a molten state, and a means for applying the adhesive to a substrate.
Hot-melt adhesives usually have relatively low cohesive strength and poor elevated temperature properties. Hot-melt adhesives are generally amorphous polyolefins, such as amorphous polypropylene, and the cohesive strength of amorphous materials is relatively low. By definition hot-melt adhesives must melt at the applicator temperature and therefore do not enjoy good elevated temperature properties.
Cross-linking of the hot-melt adhesive once it is applied to a substrate would improve both the elevated temperature properties and adhesive strength. Cross-linking can be affected by generally four types of mechanisms or methods: high-energy activation (i.e., ultraviolet light), moisture activation, heat activation, and oxygen induced coupling (i.e., drying oils). Examples of these methods are disclosed in Curing Hot Melts, I. J. Davis, 1988 TAPPI Hot-Melt Symposium and U.S. Pat. No. 4,052,527.
There are several problems presented by cross-linking hot-melt adhesives using the above methods. The majority of these methods are slow or unpredictable and require the adhesive to be open to exposure such as by ultraviolet light, moisture, or air and therefore, the adhesive could not be between two substrates. Additionally, due to the type of substrate upon which the hot-melt adhesive is applied, certain cross-linking treatments are not desirable. For example, some substrates are sensitive to treatments such as ultraviolet light treatment or moisture treatment. It would, therefore, be very desirable to obtain a hot-melt adhesive that has improved elevated temperature properties and cohesive strength while avoiding the above problems.